1. Field of the Invention
The present invention relates to a method of removing a metal impurity contained in a hydrofluoric-acid-containing chemical solution used in the steps in manufacturing a semiconductor device.
2. Description of Related Arts
With an increase in integration density of a VLSI, demand for cleaning a silicon wafer surface becomes severer. As contaminants on the silicon wafer surface, fine particles, a metal, an organic material, an oxide film, and the like are known.
An oxide film is removed by etching using a hydrofluoric-acid-containing solution. However, although the oxide film can be removed by this process, when a metal impurity (copper, gold, or the like) is contained in the hydrofluoric-acid-containing chemical solution, this metal impurity is attached to a wafer to adversely affect the electrical characteristics of the wafer. For this reason, the hydrofluoric-acid-containing solution in which a metal impurity is accumulated by dipping wafers in the hydrofluoric-acid-containing solution must be regenerated and used by purifying the hydrofluoric-acid-containing solution, or the hydrofluoric-acid-containing solution must be replaced with a new one.
As a conventional method of removing a metal impurity, a distillation method, an ion-exchange resin method, and a silicon granule adsorption method (Japanese Patent Laid-Open Nos. 3-102827 and 4-286328) are used.
The distillation method is not suitably applied to purification of a solution mixture such as a hydrofluoricacid-containing chemical solution because the composition ratio of the solution changes after distillation. An ion-exchange resin method can be suitably applied to only a diluted hydrofluoric acid solution, and cannot be suitably applied to a chemical solution which contains high-concentration hydrofluoric acid or ammonium fluoride.
A method of removing a metal impurity contained in a hydrofluoric-acid-containing chemical solution performed by a silicon granule adsorption method is as follows. That is, the hydrofluoric-acid-containing chemical solution is brought into contact with silicon granules, the metal impurity in the hydrofluoric-acid-containing chemical solution is removed by adsorbing the metal impurity on the silicon granules.
In removing the metal impurity contained in the hydrofluoric-acid-containing chemical solution using the above conventional silicon granule adsorption method, when this method is used for a long time, the metal impurity adsorbed on silicon granule surfaces is oxidized by oxygen dissolved in the hydrofluoric-acid-containing chemical solution, and the metal impurity is disadvantageously dissolved in the hydrofluoric-acid-containing chemical solution again.